The present invention relates to a process for the production of liquid compatible metals. It specifically relates to the production of a surface coated metal which is suitable for use in a heat pipe using a liquid which would normally degrade the base metal. When used in a heat pipe, the resulting metal does not exhibit any deterioration even with long term operation, because of the surface coating.
Liquid compatibility of materials, particularly as related to heat pipe operation, is basically the characteristic of being completely inert to the liquid and in addition being wetted by it. The inert nature of a material is stringently tested during long life operation in a heat pipe. In that application, solubility will cause removal of the material and eventual leaks in the casing, while any chemical activity will form gaseous by-products which quickly block operation of the heat transfer cycle and cause a complete malfunction of the heat pipe. For example, the austenitic stainless steels designated as AISI 300-series in conventional use are generally regarded as fully compatible with water, the term "compatible" implying a lack of solution or corrosive chemical attack of the steel by water. When used in a heat pipe, water has been shown to react with these same stainless steels, resulting in the formation of complex solid hydroxides and the liberation of hydrogen gas. The gas then acts to block proper vapor flow in the heat pipe. The solid hydroxides may clog the pores in the wick structure. Both effects are deleterious. The action is sufficiently rapid as to liberate enough hydrogen to affect heat pipe operation measurably within a few hours after operation is started. These effects are well known to practitioners of the heat pipe art and are described in reports of contracts let by the National Science Foundation. Effects similar to those cited here for stainless steels have also been reported with nickel and low carbon steel and with fluids such as alcohols, esters and ketones.
Although water is highly desirable as a thermodynamic working fluid, its use in heat pipes has been limited because compatibility has been found in glass, copper and certain ceramic materials, including aluminum oxide. While copper can and is being used as a heat pipe material with water fluid, such devices are limited in their application due to the low strength, the relative chemical activity and high thermal expansion of copper. Copper can not, for instance, be used with an external environment such as a natural gas flame due to excessive corrosion by the combustion gases. Glass heat pipes have also been made on an experimental basis, but the inherent weakness and brittleness of the material makes its use impractical. Ceramic heat pipes have not proved practical due to high cost and the difficulty of sealing them.
It is highly desirable to use steels in heat pipe casings because of their low cost, strength, ease of forming, corrosion resistance and compatibility with other materials of construction.
It is, therefore, an object of the present invention to provide metals which are compatible with various liquids including water and a method for preparing such metals so that they have the basic characteristics of the base metal, but are inert to any action by the liquid in contact with the metal.
It is a further object of the present invention to provide heat pipes made from such liquid-compatible metals and a method for producing such heat pipes.
It is a still further object of this invention to provide a process to produce steel heat pipes for use with water which produce no perceptible gaseous or solid by-products that can hinder the long time operation of the heat pipes.
It is an additional object of this invention to provide a method of producing metals and heat pipes which are compatible with other working fluids of the group esters, ketones, alcohols, ammonia, benzene, and their derivatives. Examples of these working fluids are: acetone, methanol, ethanol, ethylene glycol, ammonia and benzene.
It is an additional objective of this invention to produce liquid compatible heat pipes made from steels having as alloying agents materials readily forming adherent liquid compatible surface layers. These alloying materials are drawn from the group aluminum, beryllium, magnesium, titanium, hafnium, zirconium, silicon, calcium, chromium and copper.